So I need a symetric coroutine example

I'm getting close to wrapping up the new PickAxe. One of the last things I need is an example of symetric coroutines for the standard library section on the Fiber class.

I have lots of asymetric examples, but I'm struggling to come up with something decent for symetric coroutines (which use transfer to pass control between themselves.) I've coded up Conway's line squeezer, but it works better using asymetric coroutines. I've tried to come up with puzzles that are best solved with symetric...